1. Field of the Invention
The present invention relates to a composite material that consists essentially of a dense continuous film and a porous body of a mixed conducting oxide and an oxygen exchange layer applied to a process of industrial, selective permeation and separation of oxygen or a membrane reactor for partial oxidization of hydrocarbon. The present invention also provides a ceramic composition suitable for composing the composite material.
2. Description of the Related Art
In recent years, there have been remarkably progressed and developed processes of industrially separating and refining a specific component by using an ion conducting material. Among them, in a process of selectively permeating oxygen from a mixture gas such as atmosphere, there are expected applications from a small-scale medical oxygen pump to a large scale atmosphere generating and refining plant. In addition, recently, there has been discussed use of a membrane reactor for partially oxidizing hydrocarbon. In this case, an oxygen mixture gas is isolated from a hydrocarbon gas by a membrane of the ion conducting material, and oxygen selectively permeates a film, and reacts with hydrocarbon.
As oxide ion conductive ceramic materials available for use in such a purpose, there are known an oxide ion conductor for conducting only oxygen ions; and a mixed conducting oxide for conducting electrons or holes as well as oxygen ions. Among them, the mixed conducting oxide can compensate for charges required for maintaining movement of oxygen ions without forming an external current circuit, and thus, is more preferable for use in oxygen separation and a membrane reactor.
That is, in order to oxygen separation by the oxide ion mixed conductor, oxygen potentials at both sides of this mixed conducting oxide can only be differentiated from each other. As a result, only oxygen permeates the mixed conducting oxide from the higher oxygen partial pressure side to the lower oxygen partial pressure side, the other gas components cannot permeate the mixed conducting oxide, and thus, selective oxygen permeation and separation can be carried out. This principle can apply to the membrane reactor. The oxygen potential on the hydrocarbon gas side is low, and thus, an oxygen component selectively permeates the mixed conducting oxide. The resultant oxygen component is consumed by oxidizing reaction of a hydrocarbon gas, and thus, the oxygen potential on the hydrocarbon gas side is maintained to be very low.
A process of oxygen separation employing the foregoing mixed conducting oxide will be described in more detail. Here, a mixture gas (raw material gas) containing oxygen such as air, which separates the mixed conducting oxide is referred to as an “incoming side”, and a side at which a desired pure oxygen gas and an oxygen enriched air are obtained is referred to as “outgoing side”. In this method, the oxygen partial pressure at the outgoing side is lower than the oxygen partial pressure at the incoming side, but its pressure difference becomes a force of driving oxygen ion diffusion. However, the oxygen gas cannot permeate the mixed conducting oxide while it is in form of oxygen molecule. When the oxygen gas is in an oxide ion state, the oxide ion diffuses in the inside of a crystal lattice of the mixed conducting oxide through an oxygen defect (its movement rate is referred to as an oxide ion diffusion rate). That is, an oxygen molecule at the incoming side is adsorbed onto a surface of the mixed conducting oxide, dissociates into two oxygen atoms, and further, receives a negative charges, thereby producing two oxide ions. When the oxide ions reach the outgoing side after they have moved the inside of the mixed conducting oxide, the negative charges are released on the mixed conducting oxide surface at the outgoing side, and are bonded to an oxygen molecule again. At these steps, an oxygen molecule is exchanged with oxide ions at the incoming side or oxide ions are exchanged with an oxygen molecule at the outgoing side. Thus, this is referred to as an oxygen exchange reaction, and its reaction rate is referred to as oxygen exchange rate.
From the foregoing, the oxygen permeation rate depends on the two factors listed below.                (I) the oxygen ion diffusion rate in the mixed conducting oxide; and        (II) the oxygen exchange reaction rate between an oxygen molecule and oxide ions on the mixed conducting oxide surface. Therefore, it is important to improve the above two rates in order to obtain a high oxygen permeation rate. As specific factors for improving the oxygen ion diffusion rate of (I), there have been discussed (a) searching a material having its high oxygen ion conductivity; and (b) filming the mixed conducting oxide. On the other hand, as specific factors for improving the oxygen exchange reaction rate of (II), there has been discussed (c) surface reforming of the mixed conducting oxide. Hereinafter, the prior art in which each of these (a) to (c) has been discussed will be described.a. Searching a Material having its High Oxygen Ion Conductivity        
As an oxygen ion mixed conducting oxide employed for oxygen separation, for example, there is known as an available candidate material, a ceramic composition of the following formula (a) disclosed in Japanese Patent Application Laid-open No. 56-92103 or a ceramic composition of the following formula (b) disclosed in Japanese Patent Application Laid-open No. 61-21717. In addition, in Japanese Patent Application Laid-open No. 6-206707, there has been proposed an ion transport permeable film having its very wide composition range as shown in the following formula (c). These compositions are oxide materials having perovskite type crystal structures.{LaxSr(1−x)}CoO3−α (‘x’ ranges from 0.1 to 0.9, and “α” ranges from 0 to 0.5)  (a) {La(1−x)Srx}{Co(1−y)Fey}0(3−δ) (‘x’ ranges from 0.1 to 1.0, ‘y’ ranges from 0.05 to 1.0, and δ ranges from 0.5 to 0)  (b) AxA′xA″x″ByB′y′B″y″03−x  (in the element periodic system adopted according to IUPAC, A and A″ are selected from the group consisting of 1, 2, and 3 families and lanthanide family of period ‘f’; A′ is selected from the group consisting of Sr, Ca, and Mg; and B, B′, and B″ are selected from transient metals in a period ‘d’, and further,0<x<1, 0<x′<1, 0<x″<1, 0<y<1, 0<y′<1, 0<y″<1, x+x′+x″=1, y+y′+y″=1, and ‘z’ is a numeric value given when a charge of a composition is neutral.)  (c) 
Requirements for these oxides having perovskite type crystal structures to indicate high oxygen permeability include: (1) a large number of oxygen holes; and (2) constantly maintained cubic simple perovskite type structure or the like (Chem. Lett., 1985, pp. 1743-46). Therefore, in order to obtain a material having its higher oxygen permeability, the material preferably includes a plenty of oxygen holes. Alternatively, there is a tendency that such perovskite type structure is unstable when the number of oxygen holes increases. Thus, it is not easy to make these two requirements compatible with each other. The degree of this difficulty is more significant in a material with a large amount of oxygen holes and with a high oxygen permeation rate. When the number of oxygen holes is increased, for example, SrCoO(2−δ) based Sr2CO2O5(or SrCO2.5, JCPDS34-1475) with a Brownmillerite type structure or SrCrO2.52 with a BaNiO3 type structure (hexagonal crystal) described in JCPD40-1018 or the like is known as a crystal structure that appears instead of a cubic crystal perovskite type, and the oxygen permeability of these materials is lower than that of cubic crystal perovskite.
The number of oxygen holes introduced into a perovskite based material varies depending on the temperature or oxygen partial pressure of use environment as well as a material composition, thus making it difficult to determine it accurately. However, from the material composition, an approximate value can be obtained by the following formula. (3−δ)=Σ{(positive ion valence)*(positive ion ratio)}/2 where summation requires all cations (positive ions) that constitutes A and B sites of Perovskite. In addition, the number of positive ion valences is 1, 2, and 3 for alkaline metals, alkali earth metals, and rare metals, respectively, and the transient metal elements are calculated by the number of valences listed below.    Valence 2; Ni, Cu, Zn, Pb    Valence 3; Cr, Fe, Co, Al, Ga, In    Valence 4; Ti, Zr, Hf, Mn, Si, Ge, Sn    Valence 5; V, Nb, Ta, Sb, Bi    Valence 6; Mo, W
In this method, when an oxygen content (3−δ) of the conventional material is calculated, SrCoO(3−δ) and SrFeO(3−δ) is 2.5, {LaxSrx}{Co(1−y)Fey}O(3−δ) ranges 2.95 to 2.55, and {La(1−x)Srx}{Co(1−y)Fey}O(3−δ) ranges from 3 to 2.5. Although a material indicating its high oxygen permeation rate is continuously searched, many of the materials that have been reported range from 2.5≦(3−δ)≦3.0. Although a material with its (3−δ) being smaller than 2.5 has been searched, a perovskite type with a cubic crystal structure cannot be maintained. Even if the cubic crystal structure is maintained, the oxygen permeation rate is low. Thus, a material of its high oxygen permeation rate cannot be obtained yet (Russian J. Electrochem., 29, 1993, pp. 1201-09, and Solid State Ionics, 96, 1997, pp. 141-51).
Thus, as a mixed conducting oxide material employed for oxygen separation, even of the value of the oxygen content (3−δ) is low, and a large number of oxygen holes is produced, there has been requested a material with its stable cubic crystal perovskite type structure and its high oxygen permeation rate.
b. Filming the Mixed Conducting Oxide
In an elementary process in which a limited rate is produced when oxygen permeates a mixed conducting oxide, as the thickness of the mixed conducting oxide is reduced, the oxide ion diffusion in the mixed conducting oxide varies from a limited rate (limited diffusion rate), and the oxygen exchange reaction on the surface of the mixed conducting oxide changes to a limited rate (surface reaction limited regimerate). Therefore, even in the case where mixed conducting oxide materials of the same compositions are employed, in the range in which the surface reaction limited regimerate is produced, as the thickness is reduced more significantly, a larger oxygen permeation quantity can be obtained.
Therefore, as a method for actually performing oxygen separation, instead of employing the oxygen ion mixed conducting oxide as a single bulk, it is believed to be effective to form and compose this conductor as a fine and gastight thin film on a porous support body (Teraoka et al., Japanese Ceramics Society Paper Journal, vol. 97, No. 4, pp. 467-72, 1989).
The mechanical strength of this composite material is maintained by a porous body portion; and the porous body functions as a support body. In addition, a thin film portion is responsible for selectively permeating and separating oxygen from a raw material gas. Here, the requirements for the porous support body will be described. In the above paper, the following three conditions for a material that forms a porous body are exemplified.
1) Intimacy with an oxygen ion mixed conducting oxide film is good.
2) In a process of producing a composite material, reaction with the oxygen ion mixed conducting oxide film does not occur, or even if such a reaction occurs, the resultant product does not reduce its oxygen permeability.
3) The thermal expansion coefficient is almost equal to that of the oxide ion mixed conducting oxide film.
The above conditions are indispensable in order to enable the mixed conducting oxide film to be formed on the porous body.
However, it is not easy to find out a material that meets these conditions (1) to (3) simultaneously. With respect to the above condition (3), there is a problem that the linear thermal expansion coefficient of a Co- or Fe-containing perovskite that is a prominent candidate of the mixed conducting oxide film is substantially large as an oxide. For example, the average linear thermal expansion coefficients ranging from room temperatures of the oxides of the following formulas (d) and (e), which is disclosed in Japanese Patent Application Laid-open No. 9-235121, to 800° C. are about 26 ppm/° C. and about 20 ppm/° C., respectively. In contrast, even magnesia that is an oxide known as having its large linear expansion coefficient is 13.4 ppm/° C., which is a far-fetched value (Y. S. Touloukian et al., Thermophysical Properties of Matter vol. 13, IFI/Plenum).(La0.2Sr0.8) (Co0.8Fe0.2)O(3−δ)  (d) (La0.2Sr0.8)(Co0.4Fe0.4Cu0.2)O(3−δ)  (e) (La1−xSrx)CoO(3−δ)  (f) (La0.6Sr0.4)CoO(3−δ)  (g) 
In addition, the perovskite type oxide has its low properties of coexistence with other oxide materials, the above condition (2) greatly limits material selection. For example, when an alumina, zirconia or the like that is a typical oxide material is employed as a support body on which an attempt is made to sinter the oxide or the like of the above formula (f) that is employed as a thin film, there occurs a problem that the alkali earth elements in perovskite are removed, thus decomposing the film or cation is dissolved from, and the oxygen permeation rate is greatly reduced.
In order to avoid such problem, Teraoka et al. proposes that a porous body is produced with the same ceramic composition as the mixed conducting oxide film. In another literature (Teraoka et al., Japanese Ceramics Society Paper Journal, vol 97, No. 5, pp. 533-38, 1989), a composite material in which the porous body and mixed conducting oxide film are oxides of the above formula (g) is fabricated.
In the other laid-opened patent or paper concerning oxygen separation, there are many cases in which any material of a porous body may be employed as long as it meets the above conditions (1) to (3). However, there are a few examples in which a dense film of the mixed conducting oxide is formed on the porous body, and a composite material is produced. As disclosed in Japanese Patent Application Laid-open No. 8-276112, even in the case where a composite material is fabricated, the porous body has the same composition as the dense film of the mixed conducting oxide. Thus, the porous body in the prior art has been eventually forced to have the same composition as the mixed conducting oxide film in order to severe requirements.
On the other hand, in the case where the porous body has the same composition as the mixed conducting oxide film, the porous body has the same sintering properties as the film. In this condition, when an attempt is made to form a gas leakage free mixed conducting oxide film, i.e., a dense film with its high density on the porous body by using a sintering approach, a fine porous body is produced simultaneously during a process of forming the film, and the porosity of the porous body is lowered. When the porosity of the porous body is lowered, there occurs a problem that the permeation rate of the gas in the porous body is lowered, and the performance of oxygen separation of a composite material is lowered.
In order to prevent the porosity of the porous body from being lowered, if the sintering temperature of the film is lowered or if the sintering time is reduced, a well fine mixed conducting oxide film cannot be produced. In this case, there occurs a problem that a supply gas component leaks the film portion during an oxygen separation process, the raw material gas enters the separated oxygen, and the purity of the obtained oxygen is lowered.
In a method in which the porous body has the same composition as the mixed conducting oxide film, it is difficult to produce a composite material with its high oxygen separation rate; and even if such material is successfully produced, its production conditions are very limited. In producing a composite material for oxygen separation caused when a dense film made of the mixture conductive material is combined with the porous body, according to the investigation of the inventor et al., there does not exist any example that discloses the presence of such technical problem earlier than application of the present invention, and therefore, its solution is not known.
The foregoing technical problem is common to a material for selective oxygen permeation and separation process utilizing the mixed conducting oxide membrane and a material for a membrane reactor such as partial oxidization of hydrocarbon. In addition, with respect to the material for the membrane reactor, the atmosphere at the hydrocarbon gas side causes a low oxygen commitment partial pressure. As a result, there occasionally occurs a problem that a mixed conducting oxide material constituting a membrane and/or a porous support body is reduced, thereby producing a volume change, and then, the material cracks and breaks in the worst case. Therefore, the mixed conducting oxide material for a chemical reactor (membrane reactor) that utilizes a membrane process is required to have its excellent breakage resistance (reduction resistance) under a low oxygen partial pressure. Although such problem is well known in the field of a solid oxide fuel cell that utilizes an ion conductor membrane, it is not well discussed in this field that utilizes the mixed conducting oxide. For example, U. Balachandran et al,. reports that, although a material of its La0.2Sr0.6Fe0.2Co0.8Ox composition having its perovskite type crystal structure breaks under a methane partial oxidization condition, a material of SrCo0.5FeOx composition having its non-perovskite type structure has its excellent reduction properties, and can be used as a methane partial oxidization membrane for a long time without breakage (U. Balachandran et al., Applied Catalysis A: General, vol 133 (1995) 19-29). However, according to the investigation of the Inventors, there does not exist any example that measure for improving reduction resistance of the mixed conducting oxide that has its perovskite type crystal structure without changing the crystal structure was disclosed earlier than an application of the present invention.
Now, the requirements for a material of the mixed conducting oxide thin film will be described here. The first requirement is that this material has its high oxide ion conductivity as discussed in factor ‘a’ because it serves as selective gas separation. In addition to this requirement, it is required that this material can be formed as an gastight thin film. The term “gastight” used here indicates that gas leakage through a film does not occur or even if such leakage occurs, it can be ignored. In order to such fine mixed conducting oxide film, it is required that the density of the film portion is as fine as about 94% or more relevant to theoretical density. However, as exemplified below, note that, even if the film is dense, gas leakage can occur, and the require density cannot be obtained.
For example, of the materials disclosed in the prior art, when a disc-shaped sintered body with 10 mm in diameter and 1 mm in thickness having the composition of the following formula (h) was produced, a dense, gastight sample was obtained. In contrast, in the composition of the following formula (i) disclosed in the prior art, as in the composition although a dense body with its density in excess of 94% was obtained at a sintering temperature of 1200° C., a plenty of fine cracks were observed on the surface of the sintered body. In addition, in measuring the oxygen permeation rate of this sample, considerable gas leakage that can be caused by cracks was detected. Although an attempt was made to prepare a sample by changing the production conditions relevant to the composition of this formula (1), no gastight sample was obtained, thus making it difficult to employ this material as a dense thin film for oxygen separation.(La0.2Sr0.8)CoO3−α  (h) SrFeO(3−δ)  (i) C. Surface Reforming of Mixed Conducting Oxide
As has been described previously, as the thickness of the mixed conducting oxide is reduced, the distance of oxygen ion diffusion is shortened. The oxygen permeation rate is increased in inverse proportion to the thickness of the mixed conducting oxide. This relationship is met in the case where the mixed conducting oxide is sufficiently thick, that is, in the case where oxygen ion diffusion is a limited diffusion rate that limits an oxygen permeation rate. When the mixed conducting oxide is thinly filmed, the oxygen exchange reaction on the mixed conducting oxide surface (surface reaction limited regimerate) limits the oxygen permeation rate. Even if the thickness of the mixed conducting oxide is equal to or smaller than predetermined thickness, it is expected that the oxygen permeation rate does not increase.
In order to overcome the above limited surface rate, there have been reported inventions in which a metal or its oxide typically employed as an electrode material of a solid electrolytic fuel cell or a catalytic material for methane gas partial oxidization is mixed in a mixed conducting oxide or is added to the surface of the conductor, thereby improving the oxygen permeation rate. For example, there have been reported that metal elements of Ag, Pd, and Pt families are impregnated in a mixed conducting oxide that consists of a composition of Sr(1+x)/2La(1−x)/2Co1−xMexO3−δ (Me is at least one element selected from Fe, Mn, Cf, and V) (Japanese Patent Application No. 61-304169); and that a catalyst consisting of a metal such as Ag, Pt, Pd, or PrO2 or its oxide is coated on one surface of the mixed conducting oxide (Japanese Patent Application Laid-open No. 7-240115). However, in as far as the inventor et al., discussed, in the case where a perovskite type oxide discussed in the present invention was employed as a mixed conducting oxide, even if a metal catalyst layer such as Pt, Ag, or Pd or a metal oxide layer such as PrO2 was added to its surface, an increase in oxygen permeation rate was not observed.